A New Age for Astronomy: Strangest telescopes in the world - Why are
astronomers tracking exotic objects in space by trailing strings under
ships and digging holes in Antarctic ice?

New Scientist vol 141 issue 1914 - 26 February 94, page 35

Far below the foam and the freighters, in the pitch blackness and extreme
pressure of the deep Pacific, ghostly particles stream from the ocean
floor. These particles are high-energy neutrinos: subatomic hints of
apocalyptic events occurring millions of light years away.

And dangling from one of the longest pieces of string in the world, off
the coast of Hawaii, is the first of a new generation of huge telescopes
specially designed to explore the world of high-energy neutrinos. This is
DUMAND2, and it has three brothers and sisters round the world: AMANDA at
the South Pole, NESTOR in the Mediterranean near Greece, and NT-36 in
Siberia's Lake Baikal.

With their long strings of light-detecting devices, lowered into deep
water or ice to wait for the telltale flashes that signal a neutrino, none
of them looks remotely like a conventional telescope. Each of them will
literally look 'downwards,' not skywards, to see evidence of neutrinos. If
they looked upwards, they would be overwhelmed by the steady rain of
neutrinos caused by showers of cosmic rays.

By looking downwards, they can use the Earth as a filter to shield them
from this radiation. From their location thousands of feet below the
surface of water or ice, these telescopes will see evidence of neutrinos
coming the other way, through the Earth itself. As these neutrinos pass
through the planet, they interact with atoms, creating muons - positive or
negative particles with a mass much greater than an electron's. If these
muons happen, in turn, to pass through something dense but transparent,
such as water or ice, they emit flashes of light called Cherenkov
radiation. When they do, the four neutrino telescopes will be lying in
wait.

The only flashes these particular astronomers are interested in are those
caused by high-energy neutrinos coming from deep space, from active
galactic nuclei (AGNs) and other exotic objects. What distinguishes these
from any other sort is their direction. A neutrino moving upwards from the
Earth's crust is almost certainly from deep space. This is why the new
telescopes have multiple strings of phototubes, parallel to each other. A
neutrino's direction can be 'plotted' by the 'line' it makes as it - or to
be strictly accurate, the muon - passes through the water or ice,
interacting with different phototubes on different strings. The paths of
the neutrino and the muon are expected to be no more than a degree apart,
so muons offer a reliable way to link a particular neutrino to a
particular celestial object.

Precise timing of the interactions is essential to know if the
interactions on a line were caused by the same particle, and were not
simply part of the random photon 'noise' from interference such as
bioluminescence (the production of light by organisms, here bacteria in
the ocean), potassium-40 decay or atmospheric neutrinos. Once the
neutrino's path has been plotted, the scientists can trace that line back
to its point of origin - say, an AGN. And they hope to 'map' AGNs and
other high-energy neutrino sources across the sky by exploiting the
Earth's rotation: and as the Earth rotates, the neutrino telescope
automatically 'scans' the sky.

High-energy neutrinos have been detected on Earth before by about a dozen
underground particle detectors. Most have been observed by the
Irvine/Michigan/Brookhaven (IMB) observatory near Cleveland. IMB has
'seen' muons caused by neutrino interactions within the Earth rising at
almost the speed of light. But the real challenge of charting the neutrino
sky requires a much larger detector. IMB covers some 400 square metres;
DUMAND2 alone will extend 20 000 square metres. And, funds permitting, its
successor, DUMAND3, is expected to cover about a square kilometre.

High-energy neutrino telescopes could well extend the horizons of other
scientists, too. Because neutrinos would be the most likely particles to
interact with atoms in high-density parts of the planet, the neutrino flux
from within the Earth should reflect the planet's internal structure. It
might even be possible to use high-energy neutrinos to 'CAT scan' the
Earth's core and mantle, and perhaps to trace convection currents that
drive plate tectonics.

And particle physicists may want to join the party, too. 'We're talking
about neutrinos at energies we have no hope of producing on Earth,
particularly in the post-SSC days,' says John Learned of the University of
Hawaii at Manoa, who is leading the DUMAND2 project. The Superconducting
Super Collider (SSC), a giant particle accelerator which was 'killed' late
last year by the US Congress, would have generated scientifically 'useful'
neutrino beams in the range 1 to 2 teraelectronvolts.

But the sky is also a 'particle accelerator' that almost certainly swarms
with neutrinos in the gigaelectronvolt and teraeletron-volt ranges - and
beyond, up to energies 100 million times greater than any terrestrial
accelerator. Such high-energy neutrino sources might help physicists test
the theory of neutrino oscillations, which postulates that three types of
neutrinos - the electron, muon and tau neutrino - can turn into each
other. Only the existence of the electron and muon neutrinos has been
proven; perhaps high-energy neutrinos from space will help to verify the
tau neutrino, too.

In December, scientists working on DUMAND2 (less elegantly known as the
Deep Underwater Muon and Neutrino Detector) lowered a 436-metre string of
24 phototubes into the ocean west of Keahole Point on the Big Island of
Hawaii. The string, which is taller than the Eiffel Tower, floats
vertically in the water, anchored to cables from a 'junction box' 4760
metres down on the ocean floor. The junction box transmits the strings'
observations - muon flashes - as laser pulses along a 30-kilometre
fibreoptic cable to computers on the island.

The results were patchy: the system gathered 10 hours of optical data
before expiring, apparently because a temperature-regulating device on the
laser failed. But things went well enough to please the project leader,
Learned. By 16 December, the first components of the DUMAND2 array were in
place. 'With the acoustical data being collected now, the DUMAND2
collaboration has begun to operate the world's first deep ocean neutrino
telescope,' Learned told his colleagues in an e-mail message from the
ship's computer. Assuming all goes well, they hope to lower the remaining
two strings on a further expedition, by mid-1994.

DUMAND2, which Learned and his colleagues have been planning since the
mid-1970s (DUMAND1, a single prototype short string, was lowered in 1987),
is funded by the US Department of Energy, the US National Science
Foundation, the Swiss National Science Foundation, as well as the Japanese
ministry of education, Mombusho, and various German sources. So far, $6
million of the projected $10 million total costs has been received.
Eventually, the DUMAND2 array should consist of nine strings with 216
phototubes, arranged in a vertical octagonal shape and linked to the
junction box. According to one of the DUMAND2 team, Peter Grieder of the
University of Bern in Switzerland, this set-up could generate 6 billion
bytes of information a week even after data filtering and compression. The
next generation, DUMAND3, should start in 1996 if all goes according to
schedule, and if the estimated $80 million funding is available. It will
comprise approximately 8000 phototubes and generate about 40 times as much
data as DUMAND2.

Meanwhile, half a world away, other researchers have been pursuing a
radically different approach with the Antarctic Muon and Neutrino Detector
Array (AMANDA). Near the US polar station at zero degrees latitude, where
the wind-chill may be -30 degreesC or below on a 'summer' day and often
falls to about -70 degreesC, the Antarctic experimenters fire jets of hot
water into the polar ice to drill deep holes. In December, while their
DUMAND2 counterparts were bobbing on the waves, the AMANDA phototubes were
lowered a thousand metres into ice that has a surface temperature of
around -55 degreesC.

'The deeper you go, the more transparent the ice becomes,' says Buford
Price, one of the AMANDA team from the University of California at
Berkeley. This phenomenon occurs because air bubbles - which interfere
with clarity - are squeezed out by the extreme pressures at great depth.
'At the surface it's like looking through snow, then a few metres lower
it's like looking through ice cubes. And below a few hundred metres, there
are literally no bubbles left. It's as clear as being in water that depth
in Bermuda.'

Pure coincidence

According to Francis Halzen, who joined from the University of Wisconsin
at Madison, it was pure coincidence that the two teams were both
assembling their prototypes in December. 'We have done very well in
avoiding cutthroat competition,' he says. There is a sense of competition,
but the two teams exchange research findings with each other and with the
groups working on NESTOR and NT-36.

DUMAND2 and AMANDA have common roots: Learned and Halzen collaborated on
an early proposal for an ice experiment in the late 1980s, though Learned
is no longer directly involved with AMANDA. After initial tests of ice
clarity in Greenland in 1990, AMANDA scientists built the first,
small-scale prototype Antarctic detector in 1991 at the South Pole. Would
it reliably transmit the Cherenkov radiation? The results were 'very
encouraging', they reported in Nature back in September 1991. The ice
transmitted signals farther than 18 metres. Now, during the 24-hour
daylight of the Antarctic summer, they are installing a larger prototype
array near the US base that vaguely resembles a lunar settlement with a
huge geodesic dome surrounded by tractors, scientific instruments and
heated shacks.

The AMANDA scientists are naturally partisan: they reckon their approach
has distinct advantages over DUMAND2 and other sea-based neutrino
searches. Apart from its clarity, they believe that the ice contains
virtually no background radio - activity, unlike the interference in the
ocean. The ice also makes a conveniently solid and stable workbench and
servicing 'shop' for the scientists - who can escape the difficulties
facing the wave-tossed DUMAND2 researchers thousands of kilometres away
with their deeply submerged electronics. Unlike DUMAND2, they do not have
to route the data over many kilometres of cable. As one DUMAND2 researcher
confides: 'Having just spent a decade or two doing things in the ocean, I
prefer working almost any place else.'

As long ago as 1988, Halzen was envisaging the Antarctic ice sheet as a
giant neutrino telescope: 'While the South Pole may not be as pleasant as
Hawaii, or as convenient as Arkansas (where other reserachers have
speculated about creating a high - energy neutrino detector in an
underground mine), or even Lake Baikal, it may just turn out to be the
best place to initiate high-energy neutrino astrophysics.'

But DUMAND2's Grieder sees drawbacks to the ice. AMANDA 'offers some
intriguing advantages but faces horrendous logistics', he observes. For
one thing, the location is miserable in human terms - the highest
temperature in summer at the South Pole is, on average, close to - 18
degreesC, and biting winds can quickly freeze skin.

And these hardships still do not guarantee AMANDA greater scientific
success. The detector will not be as deep as DUMAND2 is in the Pacific,
mainly because it is harder to cut through ice than to lower phototubes
into water. So the polar neutrino observatory is not as well shielded from
atmospheric muons as DUMAND2. Just to make matters worse, the diameter of
the cores could limit the size of the phototubes that can be sunk.

The real debate, however, may be whether the Universe has sources of
high-energy neutrinos which are intense enough to be detected by the
experimental arrays as large as they are expected to become in the next
few years, with muon-intercepting areas of 20 000 square metres. 'Whether
the (DUMAND2) array will be large enough to do neutrino astronomy is the
question,' says Henry Crawford, a physicist at the University of
California at Berkeley who is a DUMAND2 associate. 'We really don't know
how strong the (high-energy neutrino) sources (in the sky) will be.'

Background 'noise' makes it especially difficult to detect high-energy
neutrinos with the current size of the detectors. Still, the worst fears
haven't come true - yet. Originally, DUMAND2 researchers were concerned
about the background noise of photons emitted by two sources:
bioluminescent creatures and Cherenkov radiation from the beta decay of
potassium-40 in seawater. But background noise from the decay can be
largely eliminated by checking for coincident signals at adjacent
phototubes, they say. Bioluminescence isn't a big worry, either; in the
oceans it is normally triggered by turbulence, but local sea currents are
weak. And any anxieties about contaminants settling on the phototubes have
also evaporated. In a previous study by the DUMAND2 team, one phototube
was lost for a year and a half at a depth of 5000 metres, then recovered
in a condition that Learned described as 'bloody spotless.'

Neutrino watch

Money could help decide whether the ultimate superdetector is based in
water or ice. Learned acknowledges that they might not see anything very
interesting astronomically with DUMAND2. 'The problem is that nowadays,
funding agencies want to know what you're going to find before you find
it. Can you imagine what would have happened if Christopher Columbus had
had to fill out a 'work breakdown structure' form?'

In the end, both DUMAND2 and AMANDA, and their counterparts NESTOR and
NT-36, may end up being used to create a global high-energy neutrino
telescope - one that, by pooling data from observations at different
locations, becomes the equivalent of a neutrino telescope as big as the
Earth. That way, says Learned, there would be 'all-sky' coverage 24 hours
a day. And, by precisely timing when particles arrive at different
stations, it would be possible to locate more precisely the emitting
source of, say, a supernova or a millisecond pulsar. A similar technique
has been used to locate gamma-ray sources.

NESTOR, lurking off the Greek coast, and NT-36 in Lake Baikal might also
play a part in this global neutrino watch. The Lake Baikal experiment,
which has been in operation since April 1993, involves a prototype
neutrino detector called NT-36 (it uses 36 phototubes). Participants
include the Institute of Nuclear Research in Moscow and universities in
Moscow, Tomsk and Irkutsk, plus foreign collaborators from Germany and
Hungary. The NT-36 phototubes are suspended on three separate strings,
supported by a metal framework which looks like an an umbrella, as deep as
1.1 kilometres. Like the other neutrino observatories, NT-36 can be
expanded in steps. Eventually, the Russians hope to have eight array
strings with a total of 192 phototubes, a detector that will be known as
NT-200.

Each 37-centimetre phototube can detect a flash as short as two-billionths
of a second. It can spot muons moving at relativistic speeds, and can also
look out for magnetic monopoles - hypothetical particles which are
postulated to exist as isolated north or south magnetic poles - and
nuggets of 'strange' quarks. By last autumn, the Baikal experiment had
detected 30 million muons, most of them from atmospheric neutrino events
(see Science, 15 January). However, about 1 in 10 000 appear to be moving
upwards and may be the result of neutrinos passing through the Earth. That
exciting possibility will get closer attention when the Russians install a
96-photomultiplier array next spring. But Lake Baikal is only 1.4
kilometres deep, so it does not provide the shielding the ocean provides
for DUMAND2 and NESTOR.

And off the town of Pylos in southwest Greece, NESTOR (Neutrinos from
Supernova and TeV Sources, Ocean Range) is making full use of its
advantages. Leonidas Resvanis, a physicist at the University of Athens, is
in charge of a team of researchers from Athens, Moscow and Florence, which
in 1991 started work on installing detectors and using them to measure the
atmospheric muon flux. By 1997, the team plans to build a hexagonal array
of seven towers, each of them about 250 metres tall and about 3800 metres
deep.

But what kind of astronomy might these telescopes do? One source of
high-energy neutrinos is intense gamma-ray sources such as those now being
studied by NASA's Compton Gamma Ray Observatory. Two gamma-ray sources in
the teraelectron-volt range are already known: the AGN Markarian 421 and
the pulsar within the Crab Nebula. Vic Stenger of the University of Hawaii
and co-principal investigator on DUMAND2, has explored the likelihood of
detecting high-energy neutrinos from AGNs, the expanding shells of
supernovae and binary neutron-star systems. The last look least promising
because earlier reports of extremely high-energy gamma rays from binary
neutron star systems such as Cygnus X-3 have not been confirmed. Also, no
gamma rays in the teraelectronvolt or gigalectron - volt ranges were
detected from Supernova1987a, so supernovae may not be so promising
either. However, the expanding shell of a supernova might absorb protons
accelerated by any neutron star left over after the blast, and these
proton-cloud collisions might generate high-energy neutrinos.

So the most intriguing possibility remains the detection of neutrinos from
AGNs (see 'Shine on, brilliant galaxy', 18 September 1993). Active
galaxies such as quasars and blazars are the brightest known objects in
the Universe, and some are so far away that they date back to the earliest
days of the Universe. Astronomers believe that to appear so brilliant at
such a distance they must contain what Learned calls 'some special
engine'. This engine must be comparatively small in astronomical terms
because the brightness of active galaxies changes over just a few days.
But no one knows of anything so small that can, at the same time, generate
so much energy - except that rather hypothetical object, a 'supermassive'
black hole.

These black holes are massive objects that have collapsed to something
comparatively tiny, perhaps about the size of the Solar System. For years,
astrophysicists' guiding hypothesis has been that active galaxies are
newborn galaxies with a giant black hole at their core. An accretion disc
of matter swirls around it, and eventually falls inwards. Friction in the
in-swirling gas heats the material so it generates intense radiation.
Stenger explains how a black hole could produce high-energy neutrinos:
'Protons are (expected to be) accelerated by shock waves in the accreting
matter around the central black hole, or by some other mechanism. These
collide with ultraviolet photons in the surrounding medium, producing
pions and other mesons which decay to give neutrinos.' And these
high-energy neutrinos could, in turn, provide astronomers with a way of
probing the black hole's immediate surroundings.

Even if AGNs fail to generate high-energy neutrinos, the new generation of
telescopes might see other sights - perhaps bizarre ones. One possibility
is that high-energy neutrinos may be detected from the transient, puzzling
objects called gamma - ray bursters. An even more exciting possibility is
detecting a hypothetical form of dark matter. Another 'highly speculative'
possibility, Stenger says, is the detection of high-energy neutrinos
generated by cosmic topological defects, relics of the time just after the
big bang.

Perhaps the most startling proposal is to use 'neutrino tomography' to map
the Earth's insides. This idea - originally proposed in the 1970s by
Learned and others - was revived in a talk given in December last year to
the American Geophysical Union by Chaincy Kuo. Kuo, a geophysics
postgraduate at the University of California at Berkeley and her
colleagues are working with the DUMAND2 team to explore this possibility.

For decades, seismologists have mapped the Earth's innards using 'seismic'
waves. Earthquakes and nuclear blasts trigger acoustic waves that ripple
through the Earth at different speeds through areas of different density.
This is not unlike medical ultrasound in that it uses sound waves to map
internal structures. The trouble is nobody knows for sure the precise
mineral composition of the Earth's insides, or how that com-position
varies from place to place, or how sound waves move at extremely high
pressures in different places - some pressures are so high that they are
difficult to simulate in the laboratory. So seismology relies at least in
part on some rather theoretical assumptions.

Earth scans

In contrast, because neutrinos interact with matter in ways that are well
understood, they should provide a clear picture of the density variations
throughout the Earth; the greater the density, the greater the chance that
a neutrino will interact with an atom there. The neutrino flux penetrating
Earth might be used to map density variations within it. By pooling data
from different neutrino observatories, the scientists could scan the
Earth. These 'Earth scans' would take advantage of Earth's rotation,
mapping the planetary mass distribution as high-energy neutrino sources
'moved' through the 'sky' at the astronomers' 'feet'. With luck, such
research might shed light on deep - mantle processes and questions about
the core-mantle boundary, which are currently hot topics among
geophysicists and important in understanding the forces that drive plate
tectonics. Still, it's unclear at present what advantages neutrino
scanning offers over seismology.

Learned suspects it will take 'a couple of generations' before
neutrino-scanning of the Earth could work. 'I'm not sure I'll be around
when it happens.' Its success would depend heavily on the abundance of
high-energy neutrino sources in the sky, and on how large a detector it
would take to detect them. Learned stresses he's not sceptical about
geo-scanning: 'It simply will require observing millions of neutrinos, and
with a one-kilometre-square detector that could take years of
observations.'

The high-energy neutrino researchers will not be surprised if they
discover something new. 'I shall be very surprised if we're not
surprised,' observes Hugh Bradner, one of the pioneers of the field and a
professor emeritus of Scripps Institution of Oceanography in La Jolla,
California. 'In this decade,' Learned adds, 'it's almost certain we'll see
the beginning of high-energy neutrino astronomy. Even if we don't manage
to 'CAT-scan' the Earth, we're opening a new window on the Universe. And
who knows what wonders will appear? We're stepping off into terra
incognita.'

Keay Davidson is science writer for the San Francisco Examiner and
co-author (with George Smoot) of Wrinkles in Time, published in Britain by
Little, Brown.